1. Field of the Invention
This invention relates to a high strength steel sheet having an excellent stretch flangeability, and more specifically, to a high strength steel sheet having a tensile strength (TS) of at least 980 MPa, as well as well balanced tensile strength and stretch flangeability as represented by the product of the TS and the stretch flangeability (λ) (TS×λ) of 50,000 or higher. The high strength steel sheet of the present invention may find a general use in the industrial fields including automobiles, electric appliances, and mechanics.
2. Description of the Related Art
The steel sheet which is used in the field of automobiles, electricity, and mechanics by press forming is required to have an excellent strength simultaneously with an excellent ductility. As a steel sheet which satisfies such requirements, TRIP (TRansformation Induced Plasticity) steel sheets have been focus of attention. These TRIP steel sheets contain residual austenite (residual γ) which is present as the residue of the austenite (γ) structure, and when the shape of the TRIP steel sheet is changed, this residual austenite transforms into martensite under the stress, and excellent elongation is realized by the γ simultaneously with the high strength realized by the martensite. TRIP steel sheets are classified by the type of the matrix into TRIP-aided dual phase steel (TDP steel) having the matrix of polygonal ferrite, TRIP-aided tempered martensite steel (TAM steel) having the matrix of annealed martensite, TRIP-aided bainite ferrite steel (TBF steel) having the matrix of bainitic ferrite, and the like.
Among these, TBF steel has the feature that high strength is readily realized by the rigid bainite structure as well as high elongation properties. However, this TBF steel also has the drawback of inferior stretch flangeability (hole expansion property, or local ductility).
In order to obviate the problems as described above, Japanese Patent Application Laid-Open Nos. 2004-323951 and 2004-332100 propose techniques for improving hole expansion property (having the meaning equal to that of the stretch flangeability) and resistance to hydrogen enbrittlement of the TBF steel by minimizing proportion of the residual γ. These techniques have been completed based on the conventional conception that “considerable loss in the hole expansion properties is induced when martensite or residual austenite is used for the second phase”.